diff --git a/clang/lib/AST/MicrosoftMangle.cpp b/clang/lib/AST/MicrosoftMangle.cpp
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -2684,6 +2684,7 @@
   //                      ::= I # __fastcall
   //                      ::= J # __export __fastcall
   //                      ::= Q # __vectorcall
+  //                      ::= S # __attribute__((__swiftcall__)) // Clang-only
   //                      ::= w # __regcall
   // The 'export' calling conventions are from a bygone era
   // (*cough*Win16*cough*) when functions were declared for export with
diff --git a/llvm/include/llvm/Demangle/MicrosoftDemangleNodes.h b/llvm/include/llvm/Demangle/MicrosoftDemangleNodes.h
--- a/llvm/include/llvm/Demangle/MicrosoftDemangleNodes.h
+++ b/llvm/include/llvm/Demangle/MicrosoftDemangleNodes.h
@@ -67,6 +67,7 @@
   Eabi,
   Vectorcall,
   Regcall,
+  Swift, // Clang-only
 };
 
 enum class ReferenceKind : uint8_t { None, LValueRef, RValueRef };
diff --git a/llvm/lib/Demangle/MicrosoftDemangle.cpp b/llvm/lib/Demangle/MicrosoftDemangle.cpp
--- a/llvm/lib/Demangle/MicrosoftDemangle.cpp
+++ b/llvm/lib/Demangle/MicrosoftDemangle.cpp
@@ -1711,6 +1711,8 @@
     return CallingConv::Eabi;
   case 'Q':
     return CallingConv::Vectorcall;
+  case 'S':
+    return CallingConv::Swift;
   }
 
   return CallingConv::None;
diff --git a/llvm/lib/Demangle/MicrosoftDemangleNodes.cpp b/llvm/lib/Demangle/MicrosoftDemangleNodes.cpp
--- a/llvm/lib/Demangle/MicrosoftDemangleNodes.cpp
+++ b/llvm/lib/Demangle/MicrosoftDemangleNodes.cpp
@@ -107,6 +107,9 @@
   case CallingConv::Clrcall:
     OS << "__clrcall";
     break;
+  case CallingConv::Swift:
+    OS << "__attribute__((__swiftcall__)) ";
+    break;
   default:
     break;
   }
diff --git a/llvm/test/Demangle/ms-mangle.test b/llvm/test/Demangle/ms-mangle.test
--- a/llvm/test/Demangle/ms-mangle.test
+++ b/llvm/test/Demangle/ms-mangle.test
@@ -338,6 +338,9 @@
 ?vector_func@@YQXXZ
 ; CHECK: void __vectorcall vector_func(void)
 
+?swift_func@@YSXXZ
+; CHECK: void __attribute__((__swiftcall__)) swift_func(void)
+
 ??$fn_tmpl@$1?extern_c_func@@YAXXZ@@YAXXZ
 ; CHECK: void __cdecl fn_tmpl<&void __cdecl extern_c_func(void)>(void)